(1) Field of the Invention
The present invention relates to an aluminum-tin (Al-Sn) base bearing alloy which is prepared by adding tin to an aluminum matrix and to a bearing material which is made by applying the Al-Sn base bearing alloy to a backing steel by pressure welding. More particularly, the Al-Sn base bearing alloy of the invention is characterized in that the bearing alloy is improved in several properties thereof by adding various kinds of additive elements. That is, the fatigue strength is much improved by reducing the lowering of the hardness at high temperatures and, especially, by avoiding the coarsening of tin particles. Furthermore, the wear resistance of the bearing alloy is also raised in order to improve the durability relative to the shaft to be supported which has a hard and coarse surface. Accordingly, in the case that the bearing alloy of the present invention is used for the bearing devices around the crank shafts of internal combustion engines which require severe conditions, remarkable advantages can be expected.
(2) Description of the Prior Art
In recent years, the automobile internal combustion engines are required to be made compact and to have high power. Further, as the countermeasure to the regulation of exhaust gas, they must be provided with blow-by-gas recirculation devices or the like. Therefore, the use conditions for the bearing materials in the internal combustion engines have become severe at high loads and high temperatures. Under such severe conditions, the conventional bearing materials are liable to cause fatigue failure and adnormal wearing which cause several troubles.
In connection with the shafts to be brought into engagement with the bearing materials, there is a tendency to use, in place of the hitherto produced forged shafts, less expensive shafts made of spheroidal graphite cast iron or other coarse material in order to reduce the production costs. Therefore, improvements in the wear resistance, seizure resistance and fatigue resistance at high temperatures are required.
Exemplified as the Al-Sn base alloy used for making the bearings of internal combustion engines in the prior art are: Al (remainder)--Sn (10-30)--Cr (0.1-1.0); Al (remainder)--Sn (3.5-4.5)--Si (3.5-4.5)--Cu (0.7-1.3); Al (remainder)--Sn (4-8)--Si (1-2)--Cu (0.1-2)--Ni (0.1-1); Al (remainder)--Sn (3-40)--Pb (0.1-5)--Cu (0.2-2)--Sb (0.1-3)--Si (0.2-3)--Ti (0.01-1); Al (remainder)--Sn (15-30)--Cu (0.4-2); and Al (remainder)--Sn (1-23)--Pb (1.5-9)--Cu (0.3-3)--Si (1-8), in which the values in parentheses denote the percentages by weight of the component materials.
When these conventional alloys are used for the bearings of automobile internal combustion engines under severe conditions as described above, fatigue failure is sometimes caused to occur in a short time if the engines are continuously operated under heavy loads. This is considered to be due to the fact that the temperature of the lubricant oil in an internal combustion engine becomes very high during the continuous full-load running thereof, for example, the temperature of the lubricant oil in an oil pan reaches 130.degree. C.-150.degree. C., so that the temperature of the sliding surfaces of bearings also becomes very high. As the result, since the eutectic point of such alloy is about 225.degree. C. or so, the hardness of the alloy rapidly becomes low under the high temperature conditions, which causes fusion and migration of the Sn component and the fatigue strength is resultantly lowered. The inventors of the present invention have prepared an alloy, the hardness of which is not lowered at high temperatures and the Sn component of the alloy is hardly altered. The alloy was worked into the shapes of bearings for internal combustion engines and they were subjected to fatigue tests under dynamic loads at high oil temperatures. As a result, the improvement in fatigue strength was recognized, which substantiated the above-mentioned consideration.
Further, in addition to the lowering of the fatigue strength due to the loss of hardness at high temperatures as mentioned above, the coarsening of tin particles in the texture of conventional Al-Sn alloy also causes lowering of the fatigue strength. That is, the aluminum bearing material is generally formed by applying Al-Sn alloy to a backing steel through pressure welding, in which an annealing step is required after the pressure welding in order to improve the adhesive strength between both metals. The annealing is generally done at a temperature below the point at which an Al-Fe intermetallic compound deposits and the higher the treating temperature is and the longer the treating time is, the larger the adhesion strength becomes. As a matter of fact, when the conventional Al-Sn alloy is placed in a high temperature condition during annealing, the coarsening of aluminum grain boundaries and tin particles is disadvantageously caused to occur in the alloy texture. That is, when the conventional aluminum bearing alloy is subjected to annealing in order to improve the adhesive strength to the backing steel, the coarsening of tin particles is brought about, which results in the lowering of the fatigue strength of the Al-Sn alloy at high temperatures.
Further, these conventional Al-Sn bearing alloys are not good enough in wear resistance. Especially, when shafts having hard and coarse surfaces such as those made of spheroidal graphite cast iron are brought into engagement with the bearing alloys, the wear resistance is greatly lowered and fatigue failures are liable to occur, which has been a serious problem in the prior art.